1. Field of the Invention
The present invention pertains to the field of processes that yield metal carboxylates and, more particularly, metal 2-ethylhexanoates. Still more specifically, the process involves conducting an electrolytically assisted chemical reaction between a metal and a carboxylic acid in the presence of an aliphatic alcohol solvent which precipitates the metal carboxylate reaction product.
2. Description of the Prior Art
Metal 2-ethylhexanoates and other salts of aliphatic acids are used in the manufacture of products including plastics such as polyolefins, polyvinylchloride, acrylonitrile/butadiene/styrene copolymers, reinforced polyesters, polystyrene, and impact resistant polystyrene. Metal 2-ethylhexanoates are also used as stabilizers and lubricants in plastic molding. Additionally, metal 2-ethylhexanoates are used in paint, varnish, printing ink, lubricants, catalysts, and fuel additives. More recently, metal 2-ethylhexanoates have found other uses as metal-organic precursors in the preparation of high-quality films for microelectronics. These microelectronic precursors require extremely pure chemical compositions. It is often impossible to find or economically produce metal 2-ethylhexanoates having the requisite research-grade purity for microelectronics applications.
U.S. Pat. No. 2,899,232 describes a process for the preparation of metal 2-ethylhexanoates. In this process, oxides or hydroxides of Group 2 elements of the periodic table are subjected to direct fusion with melted fatty acids. The reaction is performed at high temperature, followed by cooling and milling. The products thus obtained are always contaminated with the unreacted oxides or hydroxides. It is very difficult to eliminate these impurities.
U.S. Pat. No. 2,584,041 describes another process for the preparation of oil-soluble metal 2-ethylhexanoates. Powdered metals are heated in the presence of 2-ethylhexanoic acid in the presence of water and oxygen. Mineral oil is typically used as a solvent. This reaction is characterized by a very long reaction time. After completion of the reaction, water is distilled off from the reaction mixture to leave a solution in mineral oil as the commercial product. These solutions also contain a substantial contaminant in the form of excess 2-ethylhexanoic acid, which may remain in the mineral oil up to 25% by weight. The use of ammonium salts as catalysts was suggested in European Patent Office publication 512,346 A2, however, the presence of ammonium salts precludes use of the solutions from use in most organic syntheses. European Patent Office publication 094,760 indicates that alkali metal cations can be used as catalysts, but the presence of alkali metals in solution makes the solutions unsuitable for use in microelectronic applications.
Metal carboxylates can also be produced by a double decomposition technique, as described by A. S. Shaikh and G. M. Vest, J. Amer. Ceram. Soc. V. 69, n9 682 (1986). The reaction mechanism involves a two step process which includes first preparing an ammonium soap of the carboxylic acid, then mixing the soap with a metal chloride or nitrate. The resultant metal salt is separated by extraction in the organic solvent, e.g., xylene. The double decomposition processing requires the use of expensive water-soluble metal salts and complex equipment. The desired reaction products are contaminated with reaction byproducts, and mostly 2-ethylhexanoic acid. Elimination of the excess acid requires vacuum distillation at high temperature, which results in thermal decomposition of 2-ethylhexanoates.
An electrolysis reaction is discussed by N. Kumar, D. G. Tuck, and K. D. Watson in 65 Can. J. Chem. 740 (1987). Ethylhexanoates of Mn, Fe, Ni, Co, and Cu are obtained by anodic dissolution of the respective metals in a simple cell. Platinum is used for the cathode, and the reaction with 2-ethylhexanoic acid is conducted in an acetone or acetonitrile solvent using (CH.sub.3 CH.sub.2 O).sub.4 NClO.sub.4 as an electroconductive additive. This electrolysis method cannot be utilized with some metals because such metals as bismuth and lead undergo electrolytic dissolution at the anode, but plate out at the cathode with no substantial net contribution to the metal content in solution. Additionally, the (CH.sub.3 CH.sub.2 O).sub.4 NClO.sub.4 electroconductive additive contaminates the final solution with chlorine, which can preclude use of the solution in microelectronic applications.
There remains a need for a process that is capable of yielding metal 2-ethylhexanoates in high yield and high purity for use in microelectronic applications.